T P Hughes

Coral reefs provide important ecosystem goods and services, such as fisheries and tourism, and have great aesthetic and cultural value. Until recently, the direct and indirect effects of overfishing and pollution from agriculture and land development have been the most significant causes of the accelerating degradation of coral reefs in many places, particularly in the Caribbean. These human impacts have caused ecological shifts away from the original dominance by corals to a preponderance of fleshy seaweed or other weedy non-coral species. Importantly, these changes to reefs are compounded by the more recently superimposed impacts of global climate change, including coral bleaching and the emergence of disease. Even otherwise lightly impacted reefs, such as the northern and outer Great Barrier Reef, are increasingly vulnerable to climate change. Coral reefs are in serious decline globally; an estimated 30% are already severely damaged, and close to 60% may be lost by 2030.

Coral reefs are often described (inaccurately) as fragile ecosystems in delicate balance with nature; this notion goes hand in hand with the outmoded idea from visiting colonial scientists that the tropics are benign and stable environments (Fig. 9.1). According to this perspective, humans are typically portrayed as the disrupter of nature's delicate balance. But are coral reefs stable, fragile ecosystems? The answer is no, especially at the scales most relevant to human interaction with reefs. Coral reefs are subject to a high frequency of recurrent disturbances, and they have evolved and thrive in a dynamic environment.

From a demographic perspective, a decline in coral or a fish population is the result of births (or recruitment) being exceeded by mortality. As well as increasing the mortality of corals and other organisms, human activities also have significant effects on the regenerative processes of coral reef species, such as fecundity, fertilisation success, larval development and rates of settlement and recruitment. The scale of dispersal of larvae is only beginning to be understood, and is crucial to understanding patterns of larval recruitment and recovery from large scale disturbances. Human impacts on reproduction and dispersal

Figure 9.1 An upper reef slope at Lizard I. (3-4 m depth), showing a vibrant coral assemblage, dominated by tabular and branching species of Acropora. (Photo: T. P. Hughes.)

are much less obvious and harder to measure than catastrophic mortality, but they nonetheless play a crucial role in the long term dynamics of reefs.

Two categories of stress, acute and chronic, are useful for assessing human impacts and natural disturbances. Acute disturbances act suddenly, and usually for a short time, although their impacts may have long term repercussions. Examples include a ship grounding, an oil spill, or a nuclear bomb test. Chronic impacts occur over an extended period and are often difficult to stop. For instance, subsistence overfishing in densely populated developing countries, deforestation leading to coastal runoff of nutrients and sediment, or discharge of sewage from a coastal city are ongoing, chronic disturbances. There is some evidence that recovery from some types of human impacts is more difficult or slower than recovery from natural disturbances. According to a recent review, coral assemblages suffering from chronic (usually human) impacts recovered in only 27% of cases, compared to 69% for acute impacts.

Another useful way to view impacts on reefs is to consider how human activities affect the structure of food webs. The removal of species near the top of a food chain by fishing can lead to an increase in abundance of their prey (called a top-down effect). Many reefs worldwide have been severely overfished. Megafauna such as sharks and turtles are increasingly rare worldwide, and fisheries have moved lower down the food web targeting increasing numbers of herbivores such as parrotfish. Similarly, the addition of nutrients can stimulate growth of species at the bottom of the food web (primary producers such as phytoplankton and fleshy algae). This bottom-up effect can propagate upwards in a food web by providing more food for herbivores and their predators. Top-down and bottom-up distortions of food webs typically happen simultaneously.

Can we identify reefs that are most at risk? Anticipating and preventing damage is likely to be more effective than restoration afterwards (Box 9.1). Obviously, the number of people near a reef is crucial, for example,

BOX 9.1 CORAL REEF RESTORATION

The number of restoration projects is increasing as governments and NGOs attempt to 'do something' about the worldwide decline of coral reefs. Excluding artificial reef projects, nearly two hundred coral reef restoration studies have been undertaken worldwide over the past three decades, at a combined cost exceeding US$200 million. Most of them are small scale transplant experiments, where one or two species of corals are removed from one reef and relocated at a damaged site (e.g. after a ship grounding, cyclone, or the Asian tsunami). The total area of all of these projects is less than one square kilometre, while globally the amount of reef that has been degraded in the past few decades is about 105 times greater.

Coral reefs are much more diverse and complex than other systems such as mangrove stands, grasslands, or lakes, where restoration has sometimes been possible. However, no one has been successful at artificially restoring the biodiversity or ecological functions of a coral reef at a meaningful scale. A better outcome for sustaining coral reefs will come from addressing the root causes of reef degradation and from targeted interventions that build resilience to phase-shifts (e.g. by improving land-use practices in reef catchments, by establishing alternative employment options to reduce fishing pressure, and by reducing greenhouse gas emissions).

International Coral Reef Initiative resolution on artificial coral reef restoration and rehabilitation. 2005: available at http://www.icriforum.org/library/ICRI_resolution_ Restoration.pdf [Verified 21 February 2008].

the cities of Honolulu, Miami and Jakarta have all impacted very significantly on nearby coral reefs through sewage discharge, increased sedimentation, industrial pollution, overfishing, and so on. In comparison, the land area adjoining the GBR has relatively few people, and most of the reefs are tens of kilometres offshore. In contrast, the reefs of Jamaica and elsewhere in the Caribbean are typically 100 m or less from the beach, and millions of people live next to them. Arguably, reefs at risk should be the highest priority for conservation and management efforts.

Reefs closest to the mainland or fringing populated high islands are more likely to be at risk than reefs on outer continental shelves or unpopulated oceanic atolls. For example, on the GBR inshore reefs generally have more human impacts than elsewhere (see Chapter 11). Nutrient and sediment runoff from farming activities on land have impacted many of the reefs closest to the mainland. The chemical signals in annual growth bands of century-old coral skeletons reveal a sharp increase in coastal runoff following the arrival of cattle and sheep and large-scale land clearing in the 19th century. Coastal development and recreational fishing have also significantly impacted nearshore reefs. Historical photographs of mainland reefs show vibrant stands of corals along the Queensland coast that are increasingly degraded today.

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